minor #118 Add info about security-bundle in the README (mTorres)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Add info about security-bundle in the README

Along the lines of monolog-bridge and debug-bundle, the security-bundle is needed as a dependency to show security information panel in the toolbar. This change is to document that.

This should close #96.

Also this should be improved once #90 is resolved.

Commits
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5f031ff Add info about security-bundle in the README

README.rst

@@ -50,3 +50,7 @@ logs in the profiler.
 If you are using ``VarDumperServiceProvider``, add ``symfony/debug-bundle`` as
 a Composer dependency to display VarDumper dumps in the toolbar and the
 profiler.
+
+If you are using ``symfony/security``, add ``symfony/security-bundle`` as
+a Composer dependency to display Security related information in the toolbar
+and the profiler.